Conventional displays use optical control structures (e.g., films) to maximize the luminance at the angles where viewers see the display. When the light source is efficiently coupled at large angles outside the desired view angles, optical control films can use the principle of total internal reflection to redirect the light into the desired view angle. Such optical control films may be included in displays that additionally comprise a light source and a transmissive spatial light modulator. The light source may be composed of an array of emitters, a light box, or a light emitter that is disposed at the edge of light guide. The spatial light modulator comprises an array of selectively activated pixels that can be used to form an image or pattern (e.g., text).
When using a light guide, light may be extracted from a front planar surface of the light guide by using an array of extractor elements, for example, on a back planar surface of the light guide. Extracting the light with high luminance at angles that are somewhat parallel to the front planar surface is often done more efficiently than extracting the light normal to that surface. This is often true when the light guide tapers between the edge close to the source and the edge far from the source.
The conventional optical control film is disposed between the light source and the spatial light modulator. A plurality of micro-prisms in the film face towards the spatial light modulator and are configured such that a portion of this light is redirected into a more desired range of angles using the principle of total internal reflection.
One drawback of conventional optical control films having micro-prisms is that the films contribute to the formation of Moiré patterns. Moiré patterns are an interference effect resulting from the correlation of periodic structures. In this case, periodicity is introduced by the periodicity of the micro-prism film with the periodic pixels in the spatial light modulator. Additionally, Moiré patterns can occur because of periodicities with the extractors used to couple the light out of the light guide.
What is needed, therefore, is a way to reduce or eliminate the contributions to Moiré patterns.